Video monitoring systems are ever more widespread. To transmit the video or other information, they use broadcasting systems composed of wire-based or wireless heterogeneous networks whose architecture may be complex. In this regard one of the problems which arises is the obtaining of information about the content of the video sequence transmitted without previously decompressing the compressed video stream received. The type of information envisaged may, for example, serve to guarantee the integrity of the content of video sequences during their broadcasting in a context where they may be intercepted and modified by a malicious third party. In particular, certain zones of a video sequence may be of more important interest to the user, for example the zones identifying moving objects, as opposed to zones of lesser interest, for example zones such as the ground or the sky for which a guarantee of integrity is less crucial. Another type of information useful to the user may pertain, for example, to the characteristics of the zones of interest of an image, in particular information about the size or the color of said zones.
The prior art comprises various schemes making it possible to embed an item of information within a video stream, in particular, the techniques of digital watermarking of multimedia streams such as the one described in the following reference “Combining low-frequency and spread spectrum watermarking”, SPIE proceedings series, 1999, Jiri Fridrich. This method consists in watermarking an image in its entirety and is performed on uncompressed video streams, thereby exhibiting the drawback of being complex to implement on processors with limited resources.
Other techniques making it possible to embed an item of information by watermarking in an already compressed video stream exist but they do not make it possible to identify and to process just certain zones of greater importance within an image and not the entirety of the content of the image.
For example, French patent application FR2896938 describes a method for watermarking digital data using the coefficients of a discrete cosine transform (DCT) to embed a signature within a video. The whole of the video is considered here without previously performing an analysis making it possible to determine the most critical zones, from the user's point of view.
Other problems are not solved by the prior art, such as for example:                the watermarking of an image without any modification of the zones of interest and without decompression of the video sequence,        the generation of relevant information relating to said zones of interest and their embedding in the compressed video stream, again without decompression of the video sequence,        the insertion of watermarking-based markers making it possible to verify solely the integrity of certain relevant zones within the video sequence and not the whole of the sequence as is the case more traditionally.        